Graphic data processing apparatus for producing a tone for an edge pixel and reducing aliasing effects

ABSTRACT

A tone determining apparatus for determining tones of edge pixels of vector data, is especially suitable for use within an image forming apparatus of the type writing an image on a photoconductive drum by controlling a scanning beam by pulse width modulation (PWM). The tone determining apparatus involves dividing the edge pixels of the vector data, and then transforming the area of a trailing portion into a tone using a greater contribution ratio than a contribution ratio used when transforming the area of a leading portion, whenever the edge pixel is divided in a main scanning direction. The tone determining apparatus thus eliminates fine dots otherwise appearing at the leading portion, thus providing smoother edges.

This application is a continuation of U.S. Pat. application Ser. No.07/659,559, filed Feb. 22, 1991now U.S. Pat. No. 5,299,308.

BACKGROUND OF THE INVENTION

The present invention relates to a graphic data processing apparatus forremoving the jags of edges included in vector data and, moreparticularly, to a graphic data processing apparatus for determining thetones (densities) of edge pixels of vector data and feeding thedetermined tones to a laser printer or similar output unit.

It is a common practice with computer graphics to execute antialiasingwhich allows an image to appear more attractive on a CRT (Cathode RayTube). Conventional approaches for implementing antialiasing include (1)an uniform averaging method, (2) a weighted averaging method (2) and (3)a convolutional integration method.

With the advance of so-called DTP (Desk Top Publishing) using a personalcomputer, systems of the type printing out vector images similar tothose which are handled by the computer graphics art are extensivelyused today. Typical of such systems is one using Postscript of Adbi.Postscript belongs to a family of languages usually referred to as PDLs(Page Description Languages). PDL is a programming language fordescribing a form representative of the contents of one documentincluding the text, graphics, and their arrangement and format.Regarding a character font, this type of system is implemented with avector font. Hence, even when the text is changed in magnification, thissystem prints it out in far higher quality than a system which uses abit map font (e.g. conventional word processor). Another advantageparticular to the above-stated system is that both the character fontand the graphics can be printed out in combination.

However, a laser printer applicable to such a system has a resolutionwhich is not higher than 240 dpi to 400 dpi and, like CRT of computergraphics, suffers from alias. It is, therefore, necessary to provideeven the laser printer type printing with an antialias implementation soas to produce high quality images.

When a conventional graphic data processing apparatus is implementedwith antialiasing which uses N×N subpixel devision, it cannot achievesufficient effects since it uses only moderate N from the standpoint ofprocessing time and subjective evaluation of image quality.Specifically, excessively great N would increase the calculating timewhile excessively small N would limit the effect.

A conventional graphic data processing apparatus using the uniformaveraging scheme uses only one kind of subpixel configuration such asN * M submatix in calculating the tone (luminance and density) of apixel. This brings about a problem that, depending on the inclination ofvector data, the tone produced from the actual image and the toneproduced from the subpixel configuration greatly differ from each other,resulting in insufficient antialiasing.

A graphic data processing apparatus implemented by the weightedaveraging scheme or the convolutional integration scheme is advantageousover the apparatus implemented by the uniform averaging scheme since itreduces the difference between the tone derived from the actual area andthe tone derived from the subpixel configuration and thereby enhancingthe antialiasing effect. Such an apparatus, however, consumes asubstantial period of time in calculating the area ratio and, therefore,slows down the processing.

Moreover, in a graphic data processing apparatus provided with any oneof the conventional antialiasing schemes, despite that a laser printerwhich outputs an image by electrophotographic process is substituted forCRT which plays the role of an output unit, the luminance of CRT issimply replaced with the density of the laser printer. This isundesirable since the characteristic particular to anelectrophotographic process prevents the antialiasing effect from beingfully exhibited.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to enhance theantialising effect.

It is another object of the present invention to produce a tone notgreatly different from a tone derived from the actual area ratio withoutslowing down the processing.

It is another object of the present invention to insure the antialiasingeffect by taking account of the characteristic of an electrophotographicprocess.

It is still another object of the present invention to preventlow-density dots ascribable to the characteristic of a pulse widthmodulation system from degrading the antialiasing effect, withoutreducing the processing rate.

It is a further object of the present invention to eliminate aliasingdue to low-density dots without reducing the accuracy of antialiasing.

In accordance with the present invention, a graphic data processingapparatus for producing a tone of an edge pixel of vector data comprisesa dividing section for dividing an edge pixel of vector data intosubpixels, and a varying section for varying a configuration in whichthe dividing section divides an edge pixel.

Also, in accordance with the present invention, a graphic dataprocessing apparatus for producing a tone of an edge pixel of vectordata comprises a dividing section for dividing an edge pixel of vectordata into subpixels, and a varying section for varying a size in whichthe dividing section divides an edge pixel into subpixels byapproximating the vector data to a line vector and on the basis of aninclination of the line vector.

Further, a graphic data processing apparatus for determining a tone(density) of an edge pixel of vector data by dividing the edge pixelinto subpixels and outputting the tone, the present invention selects,among a plurality of subpixel configurations, a particular subpixelconfiguration on the basis of whether or not a marginal point exists inthe edge pixel, and divides the edge pixel divided into subpixels in theparticuar subpixel configuration.

Yet, in accordance with the present invention, a graphic data processingapparatus for producing a tone of an edge pixel of vector data comprisesa dividing section for dividing an edge pixel of vector data, and a tonedetermining section for transforming, when an edge pixel is divided in aleft-and-right direction, the area of a right portion into a tone by agreater contribution ratio than the area of a left portion.

Furthermore, in accordance with the present invention, a graphic dataprocessing apparatus for producing a tone of an edge pixel of vectordata comprises a dividing section for dividing an edge pixel intosubpixels, and a tone determining section for determining a tone of anedge pixel by dividing the edge pixel into a division region which is tobe divided into subpixels and a non-division region which is not to bedivided into subpixels and on the basis of the number of subpixels ofthe division region which are covered by an image.

In addition, in accordance with the present invention, a graphic dataprocessing apparatus for producing a tone of an edge pixel of vectordata comprises a dividing section for dividing an edge pixel of vectordata into subpixels, a deciding section for determining, on the basis ofan inclination of vector data traversing an edge pixel and the kind ofan edge, which of upper, lower, left and right portions of the edgepixel an image to be painted is positioned, storing means for storing afirst, a second, a third and a fourth weighting filter to be used whenthe image to be painted is positioned in the upper portion, lowerportion, right portion, and left portion of the edge pixel,respectively, and a tone determining section for determining a tone ofthe edge pixel by selecting, on the basis of the result of decision bythe deciding means, one of the first to fouth weighting filters matchingthe result of decision.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

FIGS. 1A and 1B show conventional antialiasing;

FIGS. 2A and 2B how antialiasing implemented by a uniform averagingscheme;

FIGS. 3A and 3B show antialiasing using a weighted averaging scheme;

FIGS. 4A to 4D show specific filters applicable to the weightedaveraging scheme;

FIG. 5 shows a convolutional integration scheme using a 3×3 pixeldivision;

FIGS. 6A and 6B and 7A to 7D indicate problems particular toconventional antialising;

FIG. 8 is a block diagram schematically showing a first embodiment ofthe image forming system in accordance with the present invention;

FIG. 9 is a block diagram schematically showing a specific constructionof a PDL controller included in the embodiment;

FIGS. 10A and 10B, 11 and 12A to 12C show the principle of theembodiment;

FIG. 13 is a flowchart demonstrating a specific operation of theembodiment;

FIGS. 14 and 15 each shows the operation of the embodiment;

FIGS. 16A to 16F show antialiasing Particular to a second embodiment ofthe present invention;

FIG. 17 is a block diagram schematically showing the second embodiment;

FIG. 18 is a block diagram schematically showing a specific constructionof a PDL controller included in the second embodiment;

FIG. 19A is a flowchart representative of a specific operation of thePDL controller;

FIG. 19B shows a procedure for painting a path;

FIG. 19C is a flowchart representative of antialiasing processing;

FIGS. 20A and 20B indicate the division of a figure into line vectors;

FIG. 21 shows tones undergone antialiasing processing;

FIGS. 22A to 22D how Y, M, C and BK image data written to plain memorysections of a page memory;

FIG. 23 is a block diagram schematically showing a control system of amulti-level color laser printer;

FIG. 24 is a section showing a specific construction of the laserprinter;

FIGS. 25A and 25B show a specific construction of a yellow recordingunit;

FIGS. 26A to 26D show multi-level drive using pulse width modulation;

FIG. 27 shows various latent images each being associated with aparticular pulse width modulation level; FIG. 28 shows a toner imagerepresentative of a square ABCD shown in FIG. 20A;

FIG. 29A to 29D show antialiasing particular to a third embodiment ofthe present invention;

FIGS. 30A and 30B show the division of a figure into line vectors;

FIG. 31 is a flowchart demonstrating antialiasing particular to thethird embodiment;

FIG. 32 shows tones produced by the processing particular to the thirdembodiment;

FIGS. 33A to 33D show Y, M, C and BK image data written to plain memorysections of a page memory in the third embodiment;

FIG. 34 shows a toner image representative of a pentagon ABCDE shown inFIG. 30A;

FIG. 35 shows a toner image representative of the pentagon ABDCE andproduced by the conventional subpixel division;

FIGS. 36A to 36E show antialiasing particular to a fourth embodiment ofthe present invention;

FIG. 37 is a flowchart demonstrating the processing of the fourthembodiment more specifically;

FIG. 38 shows tones undergone the processing of FIG. 4;

FIGS. 39A to 39D show Y, M, C and BK image data written to plain memorysections of a page memory in the fourth embodiment;

FIG. 40 shows a toner image representative of the pentagon ABCDE shownin FIG. 30A;

FIGS. 41A to 41F show antialiasing particular to a fifth embodiment;

FIG. 42 is a flowchart demonstrating the antialiasing of the fifthembodiment more specifically;

FIG. 43 shows tones resulted from the processing particular to the fifthembodiment;

FIGS. 44A to 44D show Y, M, C and BK image data written to plain memorysections of a page memory in the fifth embodiment; and

FIG. 45 shows a toner image representative of the pentagon ABCDE shownin FIG. 30A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better understand the present invention, conventional graphic dataprocessing technologies will be described.

FIG. 1A shows a specific image appearing on CRT for computer graphicsand having jagged edges (generally referred to as "alias"), while FIG.1B shows the image whose alias has undergone luminance modulation toappear smoother and more attractive. Antialiasing approaches availablewith conventional graphic data processing are (1) uniform averaging, (2)weighted averaging, and (3) convolutional integration, as statedearlier.

The averaging scheme decomposes each pixel into N * M (both beingnatural numbers) subpixels, effects raster calculation at highresolution, and then determines the luminance of each pixel by averagingthe N * M subpixels. A reference will be made to FIGS. 2A and 2B fordescribing the uniform averaging scheme specifically. As shown, assumethat the edge of of an image traverses a certain pixel (in the figure,the image extends rightward and downward from the oblique line). Then,if antialiase processing is not executed, the maximum displayableluminance (e.g. kid=255 in the case of 256 tones) is assigned to thepixel of interest, as shown in FIG. 2A. When N=M=7 averaging, forexample, is used to implement antialiasing, the pixel of interest isdivided into 7 * 7 subpixels, as shown in FIG. 2B. The subpixels inwhich the image exists are counted. The number of such subpixels(twenty-eight) is divided by the total number of subpixels (forty-ninein this case) for standardization (averaging). The maximum luminance(255) is multiplied by the result of division to produce a luminance ofthe pixel of interest. In this manner, this particular approachdetermines the luminance of a pixel by taking account of how the imageis associated therewith.

The weighted averaging approach is a modified version of the uniformaveraging approach. While the uniform averaging scheme treats all thesubpixels of a single pixel by using the same weight, i.e. , it simplycounts the subpixels covered by an image, the weighted averagingapproach assigns a particular weight to each subpixel so that theinfluence on the luminance kid may depend on which subpixels are coveredby an image. The weights are provided by a filter. FIG. 3A shows aspecific filter (here, cone filter) characteristic for implementing theweighted averaging scheme. Assume that the weighted averaging scheme isexecuted with the image data shown in FIG. 2A and by using the N=M=7division. The weights of the filter are applied to correspondingsubpixels which constitute a single pixel. For example, the subpixelpositioned at the upper right corner is "2". When all the subpixels arecovered by an image, the weight provided by the filter characteristic isthe count of a subpixel. FIG. 3B shows a displayed image pattern whichis associated with the different weights of the subpixels shown in FIG.3A. In this case, 199 subpixels are covered by an image. The number ofsuch subpixels is divided by the sum of the values of the filter (here336) as in the uniform averaging scheme, and the maximum luminance ismultiplied by the result of division produce the luminance of the pixelof interest. FIGS. 4A to 4D show specific filters for implementing theabove procedure.

The convolution integration scheme determines the luminance of a givenpixel while referencing surrounding pixels. Specifically, this schemeconsiders N'×N' pixels next to and surrounding a pixel whose luminanceshould be determined as pixels which correspond to those of the uniformaveraging scheme or the weighted averaging scheme. FIG. 5 shows theconvolutional integration scheme which references 3×3 pixels by way ofexample. In the figure, the pixel whose luminance should be determinedis designated by the reference numeral 2901. An image extends rightwardan downward from the oblique line, and smeared subpixels are counted.Each pixel is divided into a 4 * 4 matrix. In this case, therefore, useis made of a 12 * 12 filter. This kind of method is successful inremoving high-frequency components from a vector image.

The antialiasing method using the N×N subpixel matrix has a problem leftunsolved when applied to a conventional graphic data processingapparatus, as follows. Namely, excessively great N would increase thecalculating time while excessively small N would limit the antialiasingeffect. Customarily, therefore, moderate N is selected by consideringthe processing time as well as the subjective evaluation of imagequality. A conventional graphic data processing apparatus implemented bythe uniform averaging method uses only one kind of subpixelconfiguration (e.g. N * M submatrix) in calculating a tone (luminanceand density). This is undesirable in that depending on the inclinationof vector data, the tone derived from the actual area and the tonederived from the subpixel configuration greatly differ from each other,resulting in insufficient antialiasing. Specifically, when theinclination of vector data is nearly vertical or nearly horizontal, theN * M submatrix is apt to produce a tone different from the actual arearatio. Assume a 3* 3 submatrix and an edge pixel which vector datatraverses with a nearly vertical inclination, as shown in FIGS. 6A and6B. Then, the probability that, among tones "0" to "9", tons "1" and "2"are produced is small while the probability that tons "3", "6" and "9"are produced is great. As a result, the tone derived from the actualarea ("4", FIG. 6B) noticeably differs from the tone ("6", FIG. 6A)derived from the subpixel configuration, whereby the expectedantialiasing effect is not achievable.

A graphic data processing apparatus using the weighted averaging schemeor the convolutional integration scheme reduces the difference betweenthe tone derived from the actual area and the tone derived from thesubmatrix and thereby enhances the antialiasing effect, compared to theapparatus implemented by the uniform averaging method. However, such anapparatus increases the area ratio calculating time and thereby lowersthe processing rate.

Moreover, in a graphic data processing apparatus implemented by any oneof the conventional antialiasing schemes, despite that CRT is replacedwith a lasr printer which outputs an image by an electrophotographicprocess, the luminance of CRT is simply replaced with the tone of thelaser printer. In this condition, the antialiasing effect is not alwaysattainable due to the characteristic of the electrophotoraphic process.Assume that a vector image shown in FIG. 7A is subjected to antialiasprocessing to produce tones shown in FIG. 7B (tones "0" to "9" in thefigure). When the resulted tones are displayed on CRT as luminances, animage as smooth as a vector image appears on the CRT due to theantialiasing effect, as shown in FIG. 7C. However, when the tones shownin FIG. 7B are used as densities and the output of a laser beam isadjusted by pulse width modulation to form a latent image, the densitydecreases at the left end (left edge) of the latent image and increasesat the right end (right) edge of the same, as shown in FIG. 7D. Thisdegrades the advantage particular to antialiasing and is ascribable tothe fact that in the case of pulse width modulation a dot is formed by alaser beam whose duration (pulse width) depends on a tone and whosereference point is the left end of a pixel. As a result, the lower thetone of a dot, the farther the position of dot as measured from theposition of an actual image. This not only prevents an image havingtones resulted from antialiasing (see FIG. 7B) from being reproducedfaithfully but also makes jaggs conspicuous.

Preferred embodiments of the graphic data processing apparatus inaccordance with the present invention will be described hereinafter.

Embodiment 1

An image forming system incorporating a first embodiment of the presentinvention which is implemented as a PDL (Desk Top Publishing) controllerwill be described. The system forms an image by transforming vector datadescribed in PDL (Page Description Language) and outputted by PDL intoimage data by the PDL controller.

As shown in FIG. 8, the image forming system has a host computer 100 forgenerating a document described in PDL (postscript language in theembodiment). A PDL controller (graphic data processing apparatus of theinvention) 200 receives PDL from the host computer 100 page by page anddevelops it into black (BK), yellow (Y) , magenta (M) and cyan (C)multi-level image data while executing antialias processing. Amulti-level color laser printer 300 prins out the multi-level image dataoutputted by the PDL controller 200. A system controller 400 controlsthe operation of the laser printer 300.

FIG. 9 shows a specific construction of the PDL controller 200. Asshown, the PDL controller 200 has a receiving unit 201 for receiving PDLfed from the host computer 100. A CPU 202 controls the storage of thereceived PD1 and executes various kinds of processing such as antialiasprocessing. The reference numeral 203 designates an internal bus. A RAM204 stores PDL transferred thereto from the receiving unit 201 over thebus 203. A ROM 205 stores an antialiasing program and other programsthereon. A page memory 206 stores multi-level Y, M, C and BK image dataundergone antialias processing. A transmitting unit 207 transfers the Y,M, C and BK image data from the page memory 206 to the laser printer300. An I/O (Input/Output) interface 208 interfaces the PDL controller200 to the system controller 400. Subpixel varying means 208 is alsoconnected to the bus 203. The CPU 202 stores PDL received by thereceiving unit 201 in the RAM 204 via the system bus 203 according tothe program stored in the ROM 205. On storing one page of PDL in the RAM204, the CPU 202 executes an antialiasing procedure with the graphicelement having been stored in the RAM 204 and thereby stores multi-levelY, M, C and BK image data in plain memory sections included in the pagememory 206. The page memory 206 has a feature data memory section inaddition to Y, M, C and BK plain memory sections. Afterwards, the datain the page memory 206 is fed to the laser printer 300 via thetransmitting unit 207.

FIGS. 10A and 10B each shows part of, i.e., one pixel G of imgae datastored in the page memory 206. Specifically, FIG. 10A presents a casewherein the pixel G is divided into 4 * 4 subpixels S1 and four of thesixteen subpixels S1 are painted. FIG. 10B presents a case wherein thepixel G is divided into eight subpixels in the vertical direction andtwo pixels in the horizontal direction and three of the sixteensubpixels S2 are painted. The pixel shown in FIG. 10A has a luminance of25% while the pixel shown in FIG. 10B has a luminance of 18.75% Sincethe real area of the hatched portion is 12.5%, dividing the pixel intodifferent numbers of segments in the vertical and horizontal directionsis sometimes higher in accuracy despite the same number of subpixels. Inthe light of this, as shown in FIG. 11, zones A, B and C are defined inmatching relation to the inclination of the line to the x and y axes. Asshown in FIG. 12A to 12C, 4 * 4 subpixels S1, 2 * 8 subpixels S3, and8 * 2 subpixels S2 are assigned to the zones A, B and C, respectively.This is successful in enhancing accurate antialias processing.

A specific operation of the PDL controller 200 will be described withreference to FIG. 13. First, the coordinates representative of the startand end points of lines such as those shown in FIGS. 10A, 10B and 11 andthe inclinations obtainable from the coordinates are stored in the RAM204 as table data beforehand. Assume that graphic data D_(oo) shown inFIG. 14 has been stored in the page memory 206. Then, the position ofthe start line L which is O and the position of the end line L which isE are respectively coincident with the Y axis of points e_(o) (X_(o),Y_(o)) and e₂ (X₂, Y₂) of the graphic data D_(oo). Hence, the Ycoordinate of the start L=0 of the graphic data D_(oo) is read (stepS601), and then curved edges are subjected to linear approximation(S602). It is to be noted that term "linear approximation" refers toprocessing which, as shown in FIG. 15, represents a curve RL by a set oflines DL. The coordinates of the start and end points of the soapproximated line and the inclination produced from the two coordinatesare stored as table data (S603). When the subpixel varying means 209processes each line L, the inclination of the approximated line isdetermined on the basis of the table data and the table data which isstored in the RAM 204 beforehand. Next, the zone to which the obtainedinclination belongs is determined (S604), and then the sizes of thesubpixels S1, S2 and S3 are determined (S605). Subsequently, scan lineconversion is executed in each of the subpixels S1, S2 and S3 inmatching relation to the inclination of the approximated line (S606).Then, a luminance is calculated on the basis of the number of paintedpixels S1, S2 and S3 (S607) and written as pixel luminance data (S608 ).Such procedure is repeated from L=0 to L=E (S609 and S610).

As stated above, the embodiment changes the size in which a pixel isdivided into subpixels, on the basis of the inclination of anapproximated line. This allows the subpixel size for the calculation ofa pixel luminance to be changed in matching relation to the inclinationof an approximated line. As a result, more effective and accurateantialiasing is achievable with the variable subpixel size than with afixed subpixel size, enhancing the quality of a resultant image.

Embodiment 2

An image forming system having a PDL controller which is implemented bythe graphic processing apparatus will be described hereinafter.

(1) Outline of Antialiasing

Referring to FIGS. 16A to 16C, three different subpixel configurations(referred to as "submatrixes" hereinafter) applicable to this embodimentare shown. Specifically, FIGS. 16A to 16C show respectively a 1 * 9submatrix applicable to a vector data inclination θ which is tan θ>9/2(i.e. 77.47°>θ>102.53°), a 9 * 1 submatrix applicable to a vector datainclination θ which is 2/9>tan θ>-2/9 (e.g. 12.53°>θ>-12.53°), and a 3 *3 submatrix applicable to the other vector data inclinations θ. When amarginal point (start or end of vector data) exists in an edge pixel,the embodiment uses the submatrix of FIG. 16C since the inclination θ ofthe vector data cannot be determined unconditionally. For example, whenthe inclination θ of the vector data is tan θ>9/2 and, therefore, nearlyvertical, tone "4" is produced by use of the 1 * 9 submatrix and thedivision shown in FIG. 16D. On the other hand, when the conventionalmethod (3 * 3 submatrix) is used to determined a tone of the same edgepixel, tone "6" is obtained, as shown in FIG. 16E. As a result, the toneresuled from the 1 * 9 submatrix is closer (or equal) to the actual tonethan the 3 * 3 submatrix. When a maginal point exists in an edge pixel,a tone is determined by the 3 * 3 submatrix, as shown in FIG. 16F.

The processing described above executes decision on the basis of theinclination θ readily obtainable from the vector data of an edge pixeland whether or not a marginal point exists. Regarding which subpixelsthe vector data traverses, the above processing uses the same decisionprinciple as the conventional processing. Hence, the processing rate issubstantially the same as the rate particular to the conventionaluniform averaging scheme. Stated another way, the processing speed isfar higher than the processing speed achievable with the weightedaveraging scheme and convolutional integration scheme.

(2) Block Diagram of Image Forming System

The illustrative embodiment transforms vector data described in PDL andoutputted by DTP into image data via the PDL controller. The systemconstruction of the embodiment will be described with reference to FIG.17.

The image forming system has a host computer 100 for generating adocument described in PDL (postscript language in the embodiment). A PDLcontroller (graphic data processing apparatus of the invention) 200receives PDL from the host computer 100 page by page and develops itinto black BK, Y, M, and cyan C multi-level image data while executingantialias processing. A multi-level color laser printer 300 prins outthe multi-level image data outputted by the PDL controller 200. A systemcontroller 400 controls the operation of the laser printer 300.

(3) Construction and Operation of PDL Controller

FIG. 18 shows a specific construction of the PDL controller 200. Asshown, the PDL controller 200 has a receiving unit 201 for receiving PDLfed from the host computer 100. A CPU 202 controls the storage of thereceived PID1 and executes various kinds of processing such asantialiasing. The reference numeral 203 designates an internal bus. ARAM 204 stores PDL transferred thereto from the receiving unit 201 overthe bus 203. A ROM 205 stores an antialiasing program and other programsthereon. A page memory 206 stores multi-level Y, M, C and BK image dataundegone antialias processing. A transmitting unit 207 transfers the Y,M, C and BK image data from the page memory 206 to the laser printer300. An I/O (Input/Output) interface 208 interfaces the PDL controller200 to the system controller 400. The CPU 202 stores PDL received by thereceiving unit 201 in the RAM 204 via the system bus 203 according tothe program stored in the ROM 205. On storing one page of PDL in the RAM204, the PUC 202 executes an antialiasing procedure with the graphicelement having been stored in the RAM 204 and thereby stores multi-levelY, M, C and BK image data in plain memory sections included in the pagememory 206. The page memory 206 has a feature data memory section inaddition to Y, M, C and BK plain memory sections. Afterwards, the datain the page memory 206 is fed to the laser printer 300 via thetransmitting unit 207.

Referring to FIG. 19A, a specific operation of the CPU 202 included inthe PDL controller 200 will be described. Receiving PDL page by pagefrom the host computer 100, the PDL controller 200 develops it into BK,Y, M and C color image data while subjecting it to antialias processing.Regarding PDL, both the graphics and texts are described in vector data,and image data are dealt with on a page basis. One page is made up ofmore than one paths each having one or a plurality of elements (graphicand text elements).

On receiving PDL, the CPU 202 determines whether or not the element is acurve vector and, if it is a curve vector, approximates it to a linevector and then registers it in a working area as a line. The CPU 202repeats this operation with all of the graphic and text elementsincluded in one path to register resulted lines in the working area on apath basis (processing 1). Subsequently, the CPU 202 sorts the linesregistered in the working area with respect to the start y axis of theline (processing 2). Then, the CPU 202 paints the path by scanning lineswhile updating the y axis (processing 3). In this embodiment, the term"scanning line" refers to a line whose thickness is less than one pixel,as distinguished from a one-pixel scanning line having a thicknesscorresponding to one pixel. For example, when a path shown in FIG. 19Bis to be painted, the CPU 202 registers the elements located at thesides which a scanning line yc traverses and the real numerical valuesof the x coordinates traversing the scanning line yc (x₁, x₂, x₃ and x₄,FIG. 19B) in AET (Active Edge Table). Since the elements are registeredin the working area in the order in which they have been registered bythe processing 1, the x coordinates crossing the scanning line yc arenot always registered in the increasing order. For example, assumingthat the line which the scanning line yc and the coordinate x₃ traversehas been processed by the processing 1 first, then x₃ is registered inAET first as an x coordinate. Therefore, after the registration in AET,the elements of the individual sides registered in AET are sorted in xcoordinate in the increasing order. Two elements as counted from thesmallest x coordinate are paired, and then the space between the twoelements is painted (specifically, painting by a one-pixel scanning linedefined by nearby scanning lines yc and yc+1). In the event of suchpainting, antialiasing is implemented by adjusting the density of eachedge pixel in conformity to the area ratio. The processed side isremoved from AET, and then the scanning line or y axis is updated. TheCPU 202 repeats this sequence of steps until it process all of the sidesregistered in AET, i.e., all of the elements constituting one path.

The CPU 202 executes the consecutive processing 1, 2 and 3 on a pathbasis and repeats them until it reaches the last path of one page.

FIG. 19C shows a specific sequence of steps representative of antialiasprocessing which is executed during the painting procedure, i.e.processing 3. Assume that a square ABCD shown in FIG. 20A is inputtedduring the processing 1, FIG. 19A. The square ABCD has the followingelements:

(a) four line vectors AB, BC, CD and DA (real number notation)

(b) colors and luminances in the square

As shown in FIG. 20B, the square ABCD is divided into five line vectorsextending in the main scanning direction (real number notation). In theembodiment, data is added to the start and end points of each of thefive line vectors, as follows:

(c) coordinates of the start point of the vector elements(above-mentioned (a)) which define the start and end points of the linevector (real number notation)

(d) inclination of the vector elements which define the start and endpoints of the line vector

(e) features of the start and end points of the line vector (right andleft edges, apexes, lines thinner than one dot, crossing points oflines, etc).

When an edge pixel is detected during the painting procedure, theantialias processing shown in FIG. 19C is executed.

Specifically, at the beginning of the subpixel painting process, whetheror not a marginal point of vector data exists in an edge pixel isdetermined. If the answer of the decision is positive, the pixel issubdivided into subpixels by use of the 3 * 3 submatrix shown in FIG.16C so as to determine subpixels which should be painted. If the answerof the decision is negative, the inclination 0 of the vector data isdetermined. Then, the pixel is divided into subpixels by use of the 1 *9 submatrix if tan θ>9/2 (see FIG. 16A), by use of the 9 * 1 submatrixif 2/9 >tan θ>-2/9 (see FIG. 16B), or by use of the 3 * 3 submatrix ifotherwise (see FIG. 16B). This is also followed by the step ofdetermining subpixels which should be painted (S1201). Such a procedureis repeated with all of the vectors which cross the scanning line (S1202). Subsequently, the tones (densities) of the individual pixels arecalculated, the first pixel on the scanning line being first (S1203).This is followed by overwriting processing for calculating the tones(densities) of the individual colors (BK, R, G and B) of the figure(S1204), although not described specifically. Afterwards, the tones ofthe individual colors are written to the page memory by a conventionalprocedure (S1205). The steps S1203 to S1205 are repeated with all of thepixels defining one line (S1206).

The CPU 202 executes the above-stated iterative sequence of steps to thelast pixel of the scanning lines (y axis) while updating the previouslymentioned content (c) by the data of (d). FIG. 21 shows the tones k ofthe square ABCD, FIG. 20A, having been produced by the antialiasprocessing. The tones k are developed into BK, Y, M and C images bypredetermined Y, M, C and BK transform processing on the basis of thecolors and luminances in the figure (data (b)), and then written asimage data to the associated plain memory sections of the page memory206. Regarding the Y, M, C and BK transform processing, the embodimenthas a Y, M, C and BK transform program as software, although not shownor described specifically. FIGS. 22A to 22D show BK, C, M and Y dataresulted from a relation C:M:Y=1:0.5:0.3 and 100% UCR (UndercolorRemoval).

(4) Construction and Operation of Laser Printer

Referring to FIG. 23, the multi-color laser printer 300 has a developingsection 301 for uniformly charging the surface of a photoconductive drumwhich will be described, exposing the charged surface by a laser beam toform a latent image, developing the latent image by a toner, and thentransferring the resulted toner image to a recording medium.Specifically, the developing section 301 has a BK developing andtransferring section 301 bk, a C developing and transferring section301c, an M developing and transferring section 301m, and a Y developingand transferring section 301y assigned to BK data, C data, M data and Ydata, respectively, as will be described in detail later. A laserdriving section 302 receives 5-bit Y, M, C and BK data, i.e., imagedensity data from the PDL controller 200 and, in response, outputs alaser beam. The laser driving section 302 has buffer memories 303y, 303mand 303c to which the 5-bit Y, M and C data are respectively applied,laser diodes 304y, 304m, 304c and 304bk for emitting laser beamsassociated respectively with the Y, M, C and BK data, and drivers 305y,305m, 305c and 305bk for driving respectively the laser diodes 304y,304m, 304c and 304bk. The BK developing and transferring section 301bk,laser driving section 202, laser diode 304bk and driver 305bk will bereferred to as a BK recording unit BKU collectively hereinafter (seeFIG. 24). Likewise, the combination of the C developing and transferringunit 301c, laser diode 304c, driver 305c and buffer memory 303c will becalled a C recording unit CU (see FIG. 24). The combination of the Mdeveloping and transferring section 301m, laser diode 304m, driver 305mand buffer memory 303m will be called an M recording unit MU (see FIG.4). Further, let the combination of the Y developing and transferringsection 301y, laser diode 304y, driver 305y and buffer memory 303y becalled a Y recording unit YU (see FIG. 24). As shown in FIG. 23, therecording units BKU, CU, MU and YU are sequentially arranged around atransport belt 306 for transporting a recording medium in this orderwith respect to an intended direction of transport. In thisconfiguration, the laser diode 304bk effects exposure first, and thelaser diode 304y effects it last. To hold image data (output of the PDLcontroller 200) during the intervals between such successive exposure,the buffer memories 303y, 303m and 303c are incorporated in the laserdriving section 302.

Referring to FIG. 24, the multi-level color last printer 300 has thetransport belt 306, and the recording units YU, MU, CPU and BKU arrangedaround the belt 306, as stated above. Cassettes 307a and 307b each isloaded with a recording medium in the form of paper sheets. Feed rollers308a and 308b are respectively associated with the cassetts 307a and307b for feeding the paper sheets one at a time. A register roller 309positions the paper sheet fed from any one of the cassettes 307a and307b. A fixing roller 310 fixes images sequentially transferred to thepaper sheet by the recording units BKU, CU, MU and YU. The paper sheetor print coming out of the fixing roller 310 is driven out to apredetermined section by a discharge roller 311. The recording units YU,MU, CU and BKU have respectively photoconductive drums 312y, 312m, 312cand 312bk, chargers 313y, 313m, 313c and 313bk for uniformly chargingrespectively the drums 312y, 312m, 312c and 312bk, polygonal mirrors314y, 314m, 214c and 314bk and motors 315y, 315m, 315c and 315bk forsteering respectively the laser beams to the drums 312y, 312m, 312c and312bk, developing devices 316y, 316m, 316c and 316bk for developingrespectively the latent images formed on the drums 312y, 312m, 312c and312bk by toners of different colors, transfer chargers 317y, 317m, 371cand 317bk for transferring the developed images or toner images to apaper sheet, and cleaning devices 318y, 318m, 318c and 318bk forremoving respectively the toner particles remaining on the drums 312y,312m, 312c and 312bk after the image transfer. CCD line sensors 319y,318m, 319c and 319bk sense respectively predetermined patterns providedon the drums 312y, 312m, 312c and 312bk to show the process conditionsof the laser printer 300, although not shown or described specifically.The operation of the laser printer 300 will be described by taking the Yrecording unit YU as an example.

FIGS. 25A and 25B show a specific construction of the exposingarrangement of the Y recording unit YU. As shown, a laser beam issuingfrom the laser diode 304y is reflected by the polygonal mirror 314y,transmitted through an f-theta lens 320y, reflected by mirrors 321y and322y, and then transmitted through a dust-proof glass 323y to reach thedrum 312y. Since the polygon mirror 314y is driven by the motor 315y atconstant speed, the laser beam is sequentially shifted along the axis ofthe drum 312y (main scanning direction). In the illustrative embodiment,a photosensor 324y is disposed in non-exposing area to sense a referencepoint of main scanning. Since the laser diode 304y is driven on thebasis of recording data (5-bit data outputted by the PDL controller200), the drum 304y is subjected to multi-level exposure associated withthe recording data. As a result, a latent image corresponding to adocument image is electrostatically formed on the surface of the drum304y which has been uniformly charged by the charger 313y. The Ydeveloping unit 316y develops the latent image by a yellow toner. Theresulted yellow toner image is transferred to a paper sheet which is fedfrom the cassette 307a (or 307b) by the feed roller 308a (or 308b) andthen transported by the belt 306 in synchronism with the formation of atoner image by the BK recording unit BKU by way of the register roller309.

The other recording units BKU, CPU and MU are constructed and operatedin the same manner as the recording unit YU except that they have a BKdeveloping device 318bk, a C developing device 316c, and an M developingdevice 316c, respectively.

(5) Multi-Level Drive by Driver

The drivers 305y, 305m, 305c and 305bk drive respectively the laserdiodes 304y, 304m, 304c and 304bk in response to 5-bit Y, M, C and BKdata which are fed thereto from the image processing device 400. Forthis purpose, use is made of pulse width modulation. The multi-leveldrive using pulse width modulation will be described with reference toFIGS. 26A to 25D. Since the drivers 305y, 305m, 305c and 305bk and thelaser diodes 304y, 304m, 304c and 304bk each has an identicalconstruction, the following description will concentrate on the driver305y and laser diode 304y by way of example.

As shown in FIG. 26A, the driver 305y has a laser ON/OFF circuit 350 forturning the laser diode 304y on and off, a pulse width modulation (PWM)circuit 351 for modulating the pulse width of an LD drive clock by the5-bit image density data (Y data in this case), and a constant currentcircuit 352 for feeding a current (LD drive current) Id to the laserON/OFF circuit 350 for driving the laser diode 304y. FIGS. 26B and 26Cshow respectively a specific construction of the PWM circuit 351 and aspecific operation thereof. As shown, 5-bit data D0, D1, D2, D3 and D4are applied to a D latch 351a. In response, the D latch 351a selects tendifferent levels inclusive of ZERO (OFF). The data D0 indicates an OFFstate of the laser beam when it is ZERO or an ON state of the laser beamwhen it is ONE.

Only when the lasr beam is in an ON state, the other four bits, i.e. ,D1, D2, D3 and D4 allow nine different pulse widths to be selected. Theadjustment of nine different pulse widths is as follows.

First, the LD drive clock is applied to delay elements 351b, 351c, 351dand 351e to generate four different signals C1, C2, C3 and C4. A NANDgate 351iNANDs the LC drive clock and the signal C1 to produce a signalA1, and then an AND gate 351m ANDs the signal A1 and the LC drive clockto produce a signal P1 whose duty is about 1/9. In the same manner,signals P2, P3 and P4 having duties of about 2/9, 3/9 and 4/9,respectively are produced from the LD drive clock and the signals C2, C3and C4. An OR gate 351q ORs the LD drive clock and the signal C1 toproduce a signal P6 having a duty of about 11/18. Likewise, signals P7,P8 and P9 having duties of about 13/18, 15/18 and 17/18, respectively,are produced from the LD drive clock and the signals C2, C3 and C4. Onthe other hand, an OR gate 351f ORs the LD drive clock and the signal C2to produce a signal P3 whose duty is bout 90%. The LD drive clock itselfis delivered as a signal P5 whose duty is about 50% . These signals P1to P9 are applied to a data selector 351g which then selects one of themin response to the image density data D1, D2, D3 and D4. An AND gate351h outputs the signal Pn (n=0 to 3) as an LD drive clock V undergonepulse width modulation only when the data DO is ONE.

FIG. 26D shows a specific construction of the laser ON/OFF circuit 350and constant current circuit 352. As shown, the laser ON/OFF circuit 350has TTL inverters 353 and 354, differential switching circuits 355 and356 operable in a toggle fashion, and resistors R2 and R3 constituting avoltage dividing circuit. This voltage dividing circuit generates avoltage VG2 which causes, when VG1>VG2, the switching circuits 355 and356 into, respectively, an ON state and an OFF state and causes, whenVG1<VG2, the switching circuits 355 and 356 into, respectively, an OFFstate and an ON state. Hence, when the LD drive clock is in an ON state,the incerter 354 generates VG1 and thereby satisfies the conditionVG1>VG2. As a result, the switching circuits 35 and 356 become ON andOFF, respectively, and thereby turn on the laser diode 304y. Conversely,when the LD drive clock is in an OFF state, the inverter 354 does notproduce an output and thereby satisfies the condition VG1>VG2. Then, theswitching circuits 355 and 356 are respectively caused into an OFF stateand an ON state to in turn turn off the laser diode 304y. The constantcurrent sure 352 has a transistor 360 and resistors R₄ and R₅ and feedsa laser drive current to the laser ON/OFF circuit 350, as statedearlier.

FIG. 27 shows specific latent images which the laser beam from the laserdiode 304y may form on the drum 312y on the basis of levels "0" to "9"(corresponding to tones "0" to "9". Level "0" is not shown in the figuresince it is representative of the absence of a dot.

FIG. 28 shows a toner image which Embodiment 2 forms on a paper sheet inresponse to the rectangle ABCD shown in FIG. 20A.

Embodiment 3

An image forming system with a PDL controller will be described as athird embodiment of the present invention in relation to the antialiasprocessing. The rest of the construction and operation of Embodiment 3is the same as Embodiment 2.

This embodiment gives consideration to the characteristic of anelectrophotographic process that an image outputted by a laser printerhas darker pixels at a right edge than at a left edge. The imageprocessing device or PDL controller, therefore, transforms, when an edgepixel is divided into right and left portions, the area of the rightportion into a tone by a greater contribution ratio than the area of theleft portion.

The antialias processing particular to this embodiment will be describedwith reference to FIGS. 29A to 29D.

For the above-stated purpose, the illustrative embodiment uses subpixelswhich are divided by a predetermined radio such that one of two nearbysubpixels which is positioned at the right-hand side is always smallerthan the other located at the left-hand side. Specifically, as shown inFIG. 29A, a pixel is divided into subpixels in the horizontal directionby a ratio of 3:2:1 from the right to the left. While the embodiment isdescribed as adopting the 3 * 3 submatrix, it is of course practicablewith any other submatrix. In addition, the specific division ratio shownin FIG. 29A is only illustrative. FIG. 29A shows the 3 * 3 subpixeldivision, i.e., the case wherein, assuming that the output unit (herelaser printer) produces ten different tones ("0" to "9"), the area to bepainted or image area is transformed into a tone. These subpixels areprovided with the same weight, i.e., "1" with no regard to the sizethereof, as shown in FIG. 29B. Hence, when the left portion of a vectorimage is to be drawn, i.e., when the edge pixel is located at theleft-hand side, the propability that the subpixels join the image at theright portion of the pixel increases. As a result, the density tends toincrease in the edge pixel located at the left edge. Conversely, whenthe right portion of a vector image is to be drawn, i.e., when the edgepixel is located at the right-hand side, the probability that thesubpixels join the image at the left portion of the pixel decreases. Asa result, the density tends to decrease in the edge pixel located at theright edge. Assume that a vector image covers a specific area of an edgepixel, as shown in FIG. 29B. Then, the vector image involves threesubpixels and, therefore, the tone is 3/9 or "3". By contrast, when useis made of the conventional 3 * 3 submatrix having a horizontal divisionratio of 1:1:1, as shown in FIG. 29C, the image involves two subpixelsand, therefore, the tone is 2/9 or "2".

When a vector image covers the left portion of an edge pixel, thesubpixel division shown in FIG. 29B will result in a lower tone than thesubpixel division of FIG. 29A, although not described specifically. Itfollows that the subpixel division of FIG. 29A allows, when an edgepixel is divided in the right-and-left direction, the area of the rightportion to be transformed into a tone by a greater contribution ratiothan the area of the left portion. Despite such a particular weightingscheme, tones can be determined as rapidly as in the conventionalaveraging scheme. FIG. 29D shows an alternative implementation fortransforming the area of the right portion of an edge pixel into a toneby a greater contribution ratio than the area of the left portion.Specifically, in FIG. 29D, the edge pixel is divided into six subpixelsin the horizontal direction and three subpixels in the verticaldirection, and tones are determined by use of a weighted filter, i.e.,by the weighted averaging scheme. The weighted matrix of FIG. 29D is asadvantageous as the matrix of FIG. 29B (tone 3/9 or "3"). However, thematrix of FIG. 29D needs multiplication for weighting and, therefore,somewhat slower in processing rate than the matrix of FIG. 29A.

Assume that a pentagon ABCDE shown in FIG. 30A is inputted. Thispentagon has the following elements:

(a) five line vectors AB, BC, CD, DE and EA (real number notation)

(b) colors and luminances in the figure

By the procedure stated earlier, the pentagon ABCDE is divided intoseven line vectors (real number notation) extending in the main scanningdirection. In this embodiment, the following data is added to the startand end points of each of the seven line vectors:

(c) coordinates of the start point of vector elements (above-mentioned(a)) defining the start and end point of a line vector

(d) inclination of vector elements defining the start and end points ofa line vector

(e) features of the start and end points of a line vector (right andleft edges, apexes, line thinner than one dot, crossing points of lines,etc).

A reference will be made to FIG. 31 for describing antialiasing which isincluded in the processing for painting one-pixel scanning line.

As shown in FIGS. 29 to 29D, the illustrative embodiment transforms thearea of the right portion of a bisected edge pixel into a tone by agreater contribution ratio than the area of the left portion. First, onepixel is divided into 3 * 3 subpixels by the horizontal division ratioof FIG. 29A so as to determine subpixels which should be painted(S2401). This operation is repeated with all of the vectors which crossthe scanning line (S2402). Subsequently, the tones (densities) of theindividual pixels on the scanning line of interest are determined by useof a filter which implements the uniform averaging scheme (antialiasprocessing) (S2403). This is followed by the calculation of tones(densities) of individual colors (BK, R, G and B) implemented byoverwriting, although not described specifically (S2404). Thereafter,the tones of the individual colors are written to the page memory(S2405). The steps S2403 to 2405 are repeated with all of the one lineof pixels (S2406).

The CPU 202 executes the above-stated iterative sequence to the lastpixel of the scanning lines (y coordiate) while updating the content (c)by the data (d). As a result of such antialias processing, the tones kof the figure shown in FIG. 30A have specific values shown in FIG. 32.The tones k are developed into BK, Y, M and C image data bypredetermined Y, M, C and BK transform processing on the basis of thepreviously mentioned colors and luminances (data (b)) and then writtento the associated plan memory sections of the page memory 206 as imagedata. FIGS. 33A to 33D show respectively BK data, C data, M data and Ydata produced by a relation C:M:Y=1:0.5:0.3 and 100% UCR.

By the construction and operation described above, the illustrativeembodiment forms a toner image shown in FIG. 34 on a paper sheet inresponse to the pentagon ABCDE shown in FIG. 30A. When use is made ofthe 1:1:1 subpixel division, the pentagon ABCDE will be turn into atoner image shown in FIG. 35. By comparing FIG. 34 (toner image of theembodiment) with FIG. 35, it will be seen that the embodiment is capableof forming a toner image while making most of the advantage of theanteialias processing. In practice, since low-density edges of a tonerimage cannot be surely reproduced due to the characteristic (defect) ofthe electrophotographic process, the advantage of this embodiment willbe further significant in an image undergone development and transfer.Of course, the weighted filter shown in FIG. 29D is as advantageous asthe transforming the area of the right portion by a greater contributionratio than the left portion.

Embodiment 4

An image forming system with a PDL controller will be described as afourth embodiment of the present invention in relation to the antialiasprocessing. The rest of the construction and operation of Embodiment 4is the same as Embodiment 2.

This embodiment divides a single pixel into a region for dividing thepixel into subpixels and a region for not dividing it into subpixelsand, based on the number of subpixels covered by an image, determines atone. This is successful in setting tone "0" for edge pixels which areapt to adversely affect the antialias processing and thereby producealias due to the characteristic of the pulse width modulation system.

The antialias processing particular to this embodiment will be describedwith reference to FIGS. 36A to 36E. As shown in FIG. 36A, thisembodiment divides a pixel (to be subjected to antialias processing)into a left or division region to be divided into subpixels and a rightor non-division region not to be done so. The division region is dividedinto 3 * 3 subpixels. How the embodiment determines a tone will bedescribed in comparison with the conventional 3 * 3 division scheme withreference to FIGS. 36A to 36E. As shown in FIG. 36B, when an imageexists in the non-division region of the pixel and not in the subpixels,the number of subpixels to be painted is 0/9 and, therefore, the tone is"0". By contrast, as shown in FIG. 36C, the number of pixels to bepained according to the conventional 3 * 3 division scheme is 1/9,resulting in tone "1". When a dot is formed by pulse width modulation onthe basis of tone "1", the dot will be isolated from the image to renderantialias processing useless since the image overlaps the pixel onlyslightly. As shown in FIG. 36D, assume that an image overlaps thedivision region of the pixel. Then, the number of subpixels to bepainted is 2/9 and, therefore, the tone is "2". By contrast, as shown inFIG. 36E, the conventional 3 * 3 division scheme would determine that3/9 subpixels should be painted and, therefore, that the tone is "3".Although the tone determined by the embodiment is slightly smaller thanthe tone resulted from the conventional scheme, the advantage ofantialias processing is preserved since all the pixels of the left edgeare provided with comparatively low tones when viewed in the subscanningdirection. Moreover, since use is not made of a weighting filter,multiplication for weighting is not necessary and, therefore, theprocessing speed is as high as the processing speed of the ordinaryuniform averaging scheme.

A reference will be made to FIG. 37 for describing antialiasing which isincluded in the processing for painting one-pixel scanning line. Asshown in FIGS. 36A to 36E, the illustrative embodiment transforms thearea of the right portion of a bisected edge pixel into a tone by agreater contribution ratio than the area of the left portion.

First, one pixel is divided into a division region and a non-divisionregion, and the division region is subdivided into 3 * 3 subpixels so asto determine the region to be painted subpixel by subpixel (S3001). Thisoperation is repeated with all of the vectors which cross the scanningline (S3002). Subsequently, the tones (densities) of the individualpixels on the scanning line of interest are determined by use of afilter which implements the uniform averaging scheme, the first pixelbeing first (antialias processing) (S3003). This is followed by thecalculation of tones (densities) of individual colors (BK, R, G and B)implemented by overwriting, although not described specifically (S3004).Thereafter, the tones of the individual colors are written to the pagememory (S3005). The steps S3003 to S3005 are repeated with all of theone line of pixels (S3006).

The CPU 202 executes the above-stated iterative sequence to the lastpixel of the scanning lines (y coordiate) while updating the content (c)by the data (d) (see Embodiment 3). As a result of such antealiasprocessing, the tones k of the figure shown in FIG. 30A, for example,have specific values shown in FIG. 38. The tones k are developed intoBK, Y, M and C image data by predetermined Y, M, C and BK transformprocessing on the basis of the previously mentioned colors andluminances (data (b)) and then written to the associated plan memorysections of the page memory 206 as image data. FIGS. 39A to 39D showrespectively BK data, C data, M data and Y data produced by a relationC:M:Y=1:0.5:0.3 and 100% UCR.

By the construction and operation described above, the illustrativeembodiment forms a toner image shown in FIG. 40 on a paper sheet inresponse to the pentagon ABCDE shown in FIG. 30A. By comparing FIG. 40(toner image of the embodiment) with FIG. 35, it will be seen that theembodiment is capable of forming a toner image while making most of theadvantage of the anteialias processing. While this embodiment has beenshown and described as subdividing the division region into 3 * 3subpixels, such a subpixel submatrix is only illustrative. Further, thethe ratio of the non-division region to the division region may beincreased, if desired.

Embodiment 5

An image forming system with a PDL controller will be described as afifth embodiment of the present invention in relation to the antialiasprocessing. The rest of the construction and operation of Embodiment 5is the same as Embodiment 2.

In this embodiment, the image processing device or PDL controllerdetermines, on the basis of the inclination of vector data traversing anedge pixel and the kind of the edge, which of an upper, lower, right andleft portions of the pixel an image occupies. Then, based on the resultof decision, the PDL controller selects one of predetermined weightingfilters to determine a tone of the edge pixel. The tones so determinedby optimal weigting filters allow an image to be outputted in anadequate tone distribution. Especially, this embodiment frees aleft-edge pixel whose right portion should be painted from aliasascribable to a low-density dot which would otherwise result from thecharacteristic of pulse width modulation.

A reference will be made to FIGS. 41A to 41F for describing antialiasingparticular to this embodiment.

In the illustrative embodiment, use is made of four weighting filterseach having a 4 * 4 matrix and having particular weights for averagingwhich are different from the weights of the others. The tone of the edgepixel produced by the weighting filter is transformed into any one often successive tones to be applied to the pulse width modulation typemulti-color laser printer 300 operable with ten successive tones.Specifically, FIGS. 41A to 41D show four weighting filters with whichthe embodiment is practicable. The filter shown in FIG. 41A is used whenan image portion to be painted is located in a left portion of an edgepixel. The filter shown in FIG. 41B is used when such an image portionis located in a right portion of an edge pixel. The filters shown inFIGS. 41C and 41D are respectively used when the image portion islocated in a lower portion and an upper portion of an edge pixel. Itshould be noted that the weights assigned to the individual filters areonly illustrative and may be replaced with any other weights so long asthey eliminate jagged edges in a print in conformity to the number oftones, dot configuration and other factors particular to a laserprinter.

FIG. 41E lists specific conditions for determining the position of animage portion in an edge pixel on the basis of the inclination of vectordata and the kind, of an edge. For example, when the inclination α ofvector data is π/2 and the kind an edge is a right edge, it isdetermined that a left portion of the edge pixel should be painted.Then, the weighting filter shown in 41A is used.

In the embodiment, whether or not the position of an image portion to bepainted should be discriminated in the up-and-down direction or in theright-and-left direction is determined depending on whether theinclination α of vector data is greater than or smaller than π/4 (or-π/4). Of course, a reference inclination other than π/4 may be used.

Referring to FIG. 41F, how the weighting filters are used will bedescribed specifically. In the figure, a pixel G₁ has an inclination αequal to or greater than 0 and smaller than π/4 and a left edge, so thatit is determined that a lower portion thereof should be painted. Hence,the filter shown in FIG. 41C is used; the resultant tone is "4 (4/19)".A pixel G₂ has an inclination α greater than π/4 and a left edge, sothat it is determined that a right portion thereof should be painted. Inthis case, the filter shown in FIG. 41B is used; the resultant tone is"0 (0/19)". A pixel G₃ has an inclination α smaller than -π/4 and a leftedge. Then, it is determined a left portion of the pixel G₃ should bepainted; use is made of the filter shown in FIG. 41A, and the resultanttone is "6 (6/16)". Further, a pixel G₄ has an angle α equal to orgreater than 0 and smaller than π/4 and a right edge, so that it isdetermined that an upper portion thereof should be painted. Then, thefilter shown in FIG. 41D is used, resulting in a tone "0 (0/20)".

A reference will be made to FIG. 42 for describing antialiasing includedin the painting procedure particular to the illustrative embodiment.

First, subpixel painting procedure is executed to divide one pixel into4 * 4 subpixels to determine a region to be painted subpixel by subpixel(S3501). This operation is repeated with all of the vectors whichtraverse the scanning line (S3502). Subsequently, in a densitydetermining procedure, which of upper, lower, right and left portions ofan edge pixel an image portion occupies is determined on the basis ofthe inclination of vector data traversing the pixel and the kind of anedge, as shown in FIG. 41E. Then, a particular filter matching theresult of decision is selected to calculate the tones (densities) of theindividual pixels on the scanning line of interest by weightedaveraging, the first pixel being first (S3503). This is followed by thecalculation of tones (densities) of individual colors (BK, R, G and B)implemented by overwriting, although not described specifically (S3504).Thereafter, the tones of the indvidual colors are written to the pagememory (S3505). The steps S3503 to S3505 are repeated with all of theone line of pixels (S3506).

The CPU 202 executes the above-stated iterative sequence to the lastpixel of the scanning lines (y coordiate) while updating the content (c)by the data (d) (see Embodiment 3). As a result of such antealiasprocessing, the tones k of the figure shown in FIG. 30A, for example,have specific values shown in FIG. 43. The tones k are developed intoBK, Y, M and C image data by predetermined YMC and BK transformprocessing on the basis of the previously mentioned colors andluminances (data (b)) and then written to the associated plan memorysections of the page memory 206 as image data. FIGS. 44A to 44D showrespectively BK data, C data, M data and Y data produced by a relationC:M:Y=1:0.5:0.3 and 100% UCR.

By the construction and operation described above, the illustrativeembodiment forms a toner image shown in FIG. 45 on a paper sheet inresponse to the pentagon ABCDE shown in FIG. 30A. As FIG. 45 indicates,this embodiment is capable of forming a toner image while insuring theadvantage of antialiasing.

In summary, it will be seen that graphics processing apparatus of thepresent invention has various unprecedented advantages, as enumeratedbelow.

(1) Antialiasing advantageous over conventional antialiasing schemes isachievable.

(2) A tone not noticeably different from a tone produced from an actualarea ratio is attained without any decrease in the processing rate.

(3) The advantage of antialiasing is preserved with consideration givento the characteristic of an electrophotographic process.

(4) Low-density dots particular to pulse width modulation are preventedfrom adversely affecting antialias processing without any decrease inthe processing rate.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. A tone determining apparatus for determiningtones of respective edge pixels, the tone determining apparatusespecially suitable for use within an image forming apparatus whichwrites an image on a photoconductive drum by scanning a beam from leftto right and uses pulse width modulation (PWM) to define areas of ahigher tone, the tone determining apparatus comprising:a dividing meansfor dividing the edge pixels into subpixels in accordance with a pixeldividing scheme, the pixel dividing scheme dividing each edge pixelinto:(A) one or more left subpixels adjacent a left side of each edgepixel, followed by (B) one or more right subpixel adjacent a right sideof each edge pixel; and b) tone determining means for determining a toneof each edge pixel by using (A) a first contribution ratio for at leastone right subpixel of the edge pixel and (B) a second contribution ratioused for at least one left subpixel of the edge pixel;wherein: 1) thefirst and second contribution ratios determine how much the at least oneright subpixel's tone contribution value and the at least one leftsubpixel's tone contribution value respectively contribute to the toneof the edge pixel; and 2) wherein the first contribution ratio isgreater than the second contribution ratio, so that:1) edge pixel, whichare at a left edge of the area of the higher tone, are provided with ahigher tone by broadened pulse widths; and 2) edge pixel, which are at aright edge of the area of the higher tone, are provided with a lowertone by narrowed pulse widths.
 2. An apparatus as claimed in claim 1,wherein said tone determining means includes:means for determining thetone of the edge pixel by using: (1) a uniform averaging method amongthe subpixels, and(2) a pixel dividing scheme which causes at least oneof the right subpixels always to be smaller in area than at least one ofthe left subpixels.
 3. An apparatus as claimed in claim 1, wherein saidtone determining means includes:means for determining the tone of theedge pixel by using a weighted averaging method which weights at leastone right subpixel more than it weights at least one left subpixel. 4.An image forming apparatus, comprising:I) a photoconductive drum; II)scanning means for scanning a beam across the photoconductive drum fromleft to right; and III) control means for using pulse width modulation(PWM) to control the scanning means to define areas of a higher tone onan image on the photoconductive drum, the image having left and rightedges encountered by the scanning beam, the control means including: a)dividing means for dividing edge pixels into subpixels in accordancewith a pixel dividing scheme, the pixel dividing scheme dividing eachedge pixel into:(A) one or more left subpixels adjacent a left side ofeach edge pixel, followed by (B) one or more right subpixels adjacent aright side of each edge pixel; and b) tone determining means fordetermining a tone of each edge pixel by using (A) a first contributionratio for at least one right subpixel of the edge pixel and (B) a secondcontribution ratio used for at least one left subpixel of the edgepixel;wherein: 1) the first and second contribution ratios determine howmuch the at least one right subpixel's tone contribution value and theat least one left subpixel's tone contribution value respectivelycontribute to the tone of the edge pixel; and 2) wherein the firstcontribution ratio is greater than the second contribution ratio, sothat:1) edge pixels, which are at a left edge of the area of the highertone, are provided with a higher tone by broadened pulse widths; and 2)edge pixels, which are at a right edge of the area of the higher tone,are provided with a lower tone by narrowed pulse widths.
 5. An apparatusas claimed in claim 4, wherein said tone determining meansincludes:means for determining the tone of the edge pixel by using:(1) auniform averaging method among the subpixels, and (2) a pixel dividingscheme which causes at least one of the right subpixels always to besmaller in area than at least one of the left subpixels.
 6. An apparatusas claimed in claim 4, wherein said tone determining meansincludes:means for determining the tone of the edge pixel by using aweighted averaging method which weights at least one right subpixel morethan it weights at least one left subpixel.